Rechargeable batteries are currently the primary power source for cell phones and various other portable electronic devices. The energy stored in the batteries is limited. It is determined by the energy density (Wh/L) of the storage material, its chemistry, and the volume of the battery. For example, for a typical Li ion cell phone battery with a 250 Wh/L energy density, a 10 cc battery would store 2.5 Wh of energy. Depending upon the usage, the energy could last for a few hours to a few days. Recharging always requires access to an electrical outlet. The limited amount of stored energy and the frequent recharging are major inconveniences associated with batteries. Accordingly, there is a need for a longer lasting, easily recharging solution for cell phone power sources. One approach to fulfill this need is to have a hybrid power source with a rechargeable battery and a method to trickle charge the battery or develop miniature power sources to directly power the cell phone or other electronic devices. Important considerations for an energy conversion device for this application include power density, energy density, size, and the efficiency of energy conversion.
Energy harvesting methods such as solar cells, thermoelectric generators using ambient temperature fluctuations, and piezoelectric generators using natural vibrations are very attractive power sources to trickle charge a battery. However, the energy generated by these methods is small, usually only a few milliwatts. In the regime of interest, namely, a few hundred milliwatts, this dictates that a large volume is required to generate sufficient power, making it unattractive for cell phone type applications.
An alternative approach is to carry a high energy density fuel and convert this fuel energy with high efficiency into electrical energy to recharge the battery or directly power the cell phone or other electronic device. Radioactive isotope fuels with high energy density are also being investigated for portable power sources. However, with this approach the power densities are low and there are safety concerns associated with the radioactive materials. This is an attractive power source for remote sensor-type applications, but not for cell phone power sources. Among the various other energy conversion technologies, the most attractive one is fuel cell technology because of its high efficiency of energy conversion and the demonstrated feasibility to miniaturize with high efficiency.
Fuel cells with active control systems and those capable of operating at high temperatures are complex systems and are very difficult to miniaturize to the 2-5 cc volume needed for cell phone application. Examples of these include active control direct methanol or formic acid fuel cells (DMFC or DFAFC), reformed hydrogen fuel cells (RHFC), and solid oxide fuel cells (SOFC). Passive air-breathing hydrogen fuel cells, passive DMFC or DFAFC, and biofuel cells are attractive systems for this application. However, in addition to the miniaturization issues, other concerns include supply of hydrogen for hydrogen fuel cells, lifetime and energy density for passive DMFC and DFAFC, and lifetime, energy density and power density with biofuel cells.
Conventional hydrogen fuel cells, DMFC and DFAFC designs comprise planar, stacked layers for each cell. Individual cells may then be stacked for higher power, redundancy, and reliability. The layers typically comprise graphite, carbon or carbon composites, polymeric materials, metal such as titanium and stainless steel, and ceramic. The functional area of the stacked layers is restricted, usually on the perimeter, by vias for bolting the structure together and accommodating the passage of fuel and an oxidant along and between cells. Additionally, the planar, stacked cells derive power only from a fuel/oxidant interchange in a cross-sectional area (x and y coordinates).
To design a fuel cell/battery hybrid power source in the same volume as a typical mobile device battery (10 cc-2.5 Wh), both a smaller battery and a fuel cell with high power density and efficiency would be required to achieve an overall energy density higher than that of the battery alone. For example, for a 4-5 cc (1.0-1.25 Wh) battery to meet the peak demands of the phone, the fuel cell would need to fit in 1-2 cc, with the fuel taking up the rest of the volume. The power output of the fuel cell needs to be 0.5 W or higher to be able to recharge the battery in a reasonable time. Most development activities on small fuel cells are attempts to miniaturize traditional fuel cell designs, and the resultant systems are still too big for mobile applications. A few micro fuel cell development activities have been disclosed using traditional silicon processing methods in planar fuel cell configurations, and in a few cases, porous silicon is employed to increase the surface area and power densities. See, for example, U.S. Patent/Publication Numbers 2004/0185323, 2004/0058226, U.S. Pat. No. 6,541,149, and 2003/0003347. However, the power densities of the air-breathing planar hydrogen fuel cells are typically in the range of 50-100 mW/cm2. To produce 500 mW would require 5 cm2 or more active area. Further, the operating voltage of a single fuel cell is in the range of 0.5-0.7V. At least four to five cells need to be connected in series to bring the fuel cell operating voltage to 2-3V and for efficient DC-DC conversion to 4V in order to charge the Li ion battery. Therefore, the traditional planar fuel cell approach will not be able to meet the requirements in a 1-2 cc volume for a fuel cell in the fuel cell/battery hybrid power source for cell phone use.
Microfabricated fuel cells, however, still have the fundamental components of large scale fuel cells, or components which perform similar functions. Among these are gas diffusion layers, catalyst supports, and electrocatalysts. Typically, a supply of hydrogen is provided by etching holes through the backside of rigid substrate. With a stacked structure as described in U.S. Pat. No. 6,541,149 and U.S. patent publications 2004/0185323, 2004/0058226, and 2003/0003347, alignment of the holes is not critical as all the holes reach to the anode. However, for any 3-D fuel cell with anodes and cathodes arranged in the same plane of the substrate, creation of the individual hydrogen access holes under each anode and alignment of the holes is critical. The presence of a large number of holes through the substrate tend to make the substrate fragile. The stresses generated by the thermal expansion and contraction of the electrolyte during the fuel cell operation can create cracks in the substrate causing serious reliability concerns. Furthermore, fabrication processes for these high aspect ratio holes through the rigid substrate and alignment under the anodes is expensive. Therefore a method and structure is needed that overcomes these issues.
Accordingly, it is desirable to provide an integrated micro fuel cell that derives power from a three-dimensional fuel/oxidant interchange having increased surface area and is provided on a substrate that is either flexible and/or porous, thereby avoiding precise alignment requirements of the holes providing fuel thereto. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.